Method and apparatus for dissipative clamping of an electrical circuit

ABSTRACT

Dissipative clamping apparatuses and methods for electrical circuits. In one aspect of the invention, In one aspect of the invention, a method includes switching a power supply input on an energy transfer element, regulating a power supply output by switching the power supply input on the energy transfer element, clamping a voltage on the energy transfer element to a clamp voltage and varying the clamp voltage in response to the power supply input. In another aspect, an electrical circuit includes a dissipative clamp circuit coupled to an input of the electrical circuit. An inductive element is coupled between the dissipative clamp circuit and an output of the electrical circuit. A switch is coupled in series with the inductive element. The dissipative clamp circuit is coupled to provide a clamp voltage across the inductive element, the clamp voltage is provided by the dissipative clamp circuit responsive to conditions at the input of the electrical circuit, the dissipative clamp circuit is coupled to maintain a voltage across the switch below a switch voltage limit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrical circuits and, morespecifically; the present invention relates to electrical circuitclamping.

2. Background Information

Electronic devices use power to operate. Switched mode power suppliesare commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highfrequency (e.g. 30 to 300 kHz) AC, using a switched mode power supplycontrol circuit. This high frequency, high voltage AC is applied to atransformer to transform the voltage, usually to a lower voltage, and toprovide safety isolation. The output of the transformer is rectified toprovide a regulated direct current (DC) output, which may be used topower an electronic device. The switched mode power supply controlcircuit usually provides output regulation by sensing the output andcontrolling it in a closed loop.

To illustrate, FIG. 1 is a schematic of a known forward power converter101. A switch Q1 103 turns on and off in response to a control 105 toprovide a regulated DC output voltage V_(OUT) 129 from an unregulated DCinput voltage V_(IN) 127. In one embodiment, control 105 and switch Q1103 are included in a switching regulator, which may be used to regulatethe output voltage V_(OUT) 129. This topology is well known and itsoperation is well documented.

Every forward converter must have a way to set the voltage on theprimary winding 107 of the transformer 109 during the time when theswitch Q1 103 is off. A popular way to set the voltage is with a clampnetwork 111 connected across the primary winding 107. The known clampnetwork 111 illustrated in FIG. 1 includes a resistor 113, a capacitor115 and a diode 117 and absorbs and dissipates parasitic energy from thetransformer 109 that is not delivered to the load 119 nor returned tothe input 121. The balance of energy into the clamp network 111 throughdiode 117 and energy dissipated in 113 determines a clamp voltageV_(CLAMP) 123 that is necessary prevent saturation of the transformer109.

FIG. 2 shows with idealized waveforms how the voltage V_(SWITCH) 125 onswitch Q1 103 is related to the input voltage V_(IN) 127 and the clampvoltage V_(CLAMP) 123. The clamp voltage V_(CLAMP) 123 must be highenough to prevent saturation of the transformer 109, but low enough tokeep the voltage V_(SWITCH) 125 below the breakdown voltage of switch Q1103.

FIG. 3 shows the relationship between V_(CLAMP) 123 and V_(IN) 127 in aknown power supply. As the input voltage V_(IN) 127 changes, the clampvoltage V_(CLAMP) 123 must be confined between the two boundaries shownin FIG. 3. The maximum voltage boundary is a straight line determined bythe breakdown voltage of switch Q1 103. The minimum voltage boundary isa curved line determined by the voltage necessary to keep thetransformer 109 from saturation.

FIG. 3 shows how the clamp voltage V_(CLAMP) 123 behaves with an RCDnetwork, such as that illustrated in clamp network 111 of FIG. 1. Whenthe power converter 101 operates in continuous conduction mode, theclamp voltage V_(CLAMP) 123 stays substantially constant in response tochanges in V_(IN) 127 at given load. The presence of leakage inductancein the transformer 109 causes the clamp voltage V_(CLAMP) 123 to changewith load 119. It is higher for greater current and lower for lesscurrent. The result is a restricted range of permissible input voltageV_(IN) 127 that is shown in the shaded region of FIG. 3.

SUMMARY OF THE INVENTION

Dissipative clamping methods and apparatuses are disclosed. In oneaspect of the invention, a method includes switching a power supplyinput on an energy transfer element, regulating a power supply output byswitching the power supply input on the energy transfer element,clamping a voltage on the energy transfer element to a clamp voltage andvarying the clamp voltage in response to the power supply input.Additional features and benefits of the present invention will becomeapparent from the detailed description, figures and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic diagram illustrating a known forward converterpower supply.

FIG. 2 is a timing diagram illustrating how the voltage on the switch isrelated to the input voltage and the clamp voltage in a known powersupply.

FIG. 3 is a diagram illustrating the relationship between the clampvoltage and the input voltage in a known power supply

FIG. 4 is a block diagram illustrating one embodiment of the generalelements of a dissipative clamp network in accordance with the teachingsof the present invention.

FIG. 5 is a diagram illustrating one embodiment of the relationshipbetween the clamp voltage and the input voltage in accordance with theteachings of the present invention.

FIG. 6 is a schematic diagram illustrating one embodiment of a powersupply using a dissipative clamp network in accordance with theteachings of the present invention.

DETAILED DESCRIPTION

Embodiments of methods and apparatuses for dissipatively clamping anelectrical circuit such as a power supply regulator are disclosed. Inthe following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

As an overview, FIG. 4 shows the general elements of one embodiment of adissipative clamp network 411 in an electrical circuit, such as forexample a power supply 401, in accordance with the teachings of thepresent invention. As shown, an input voltage V_(IN) 427 is received atan input 421. A clamp network 411 is used to clamp the voltage V_(CLAMP)423 across the primary winding 407 of a transformer 409. A switch 403 iscoupled to primary winding 407 to drive primary winding 407 in responseto a control circuit (not shown). It is appreciated that transformer 409is an inductive element and may be referred to as an energy transferelement or the like. A clamp diode D_(CLAMP) 437 provides aunidirectional path for the energy from the primary winding 407 of thetransformer 409 to enter the clamp network 411. The energy is held by anenergy storage element 435 and is lost through a dissipative element433. In one embodiment, the dissipative element 433 is programmed by asignal S₁ 439 from a sensing network 431. The sensing network 431produces the programming signal S₁ 439 from measurements of the inputvoltage V_(IN) 427, the voltage on the energy storage element 435 and areference voltage V_(REF) 441 received by the sensing network 431. Thus,in one embodiment, energy stored in the leakage inductance oftransformer 409 is dissipated in response to the input voltage V_(IN)427.

In one embodiment, the dissipative element 433 is adapted in accordancewith the teachings of the present invention, which can be viewed ashaving the effect of changing the value of the resistor 113 in the RCDclamp network 111 of FIG. 1. The control from programming signal S₁ 439from sensing network 431 adjusts the energy balance to maintain adesired locus of clamp voltage over an extended range of input voltageas illustrated in FIG. 5. As shown, in one embodiment the clamp voltageV_(CLAMP) 423 is varied substantially inversely linearly with respect tothe input voltage V_(IN) 427 in accordance with the teachings of thepresent invention. Thus, in one embodiment, the clamp voltage V_(CLAMP)423 is varied substantially independent of the power supply outputand/or leakage inductance of transformer 409.

With the variation in clamp voltage V_(CLAMP) 423 as shown, the range ofinput voltages for V_(IN) 427 is increased in accordance with theteachings of the present invention. Indeed, various embodiments of thepresent invention allow operation over an extended range of inputvoltage for V_(IN) 427 while maintaining the clamp voltage V_(CLAMP) 423at a high value within the minimum and maximum boundaries as shown inFIG. 5. The higher voltages made possible by a variable clamp voltageV_(CLAMP) 423, such as illustrated in FIG. 5, allows the use ofparasitic capacitance in the primary winding 407 and secondary windings443 to process some of the energy that otherwise would be dissipated inthe clamp circuit 411.

FIG. 6 is a schematic illustrating one embodiment of an electricalcircuit such as for example a power supply 601 utilizing a dissipativeclamp network 611 in accordance with the teachings of the presentinvention. As shown in the depicted embodiment, diode D3 637 providesthe unidirectional path for energy from the primary winding 607 of thetransformer 609 to enter the network 611 and capacitor C2 635 is theenergy storage element of the clamp network 611. Zener diode VR1 645 andcapacitor C3 647 make a stable-reference voltage V_(REF) 641. In oneembodiment, an N-channel metal oxide semiconductor (MOS) transistor Q2is the principal dissipative element 633. In another embodiment, it isappreciated that other types of dissipative elements could be used inplace of an N-channel MOS transistor such as for example p-channel MOStransistor, a bipolar transistor or the like or other future arisingtechnology performing the function. In one embodiment, the sensingnetwork in power supply 601 includes the connection of resistors R1 649,R2 651, R3 653 and R4 655 with transistor Q3 657. The voltage on thegate of transistor Q2 633 is the programming signal S₁ 639 that adaptsthe dissipation to achieve the desired characteristic of operation.

In one embodiment, resistors R2 651 and R4 655 form a voltage dividerthat applies a scaled value of the sum of the input voltage V_(IN) 627received at input 621 and the reference voltage V_(REF) 641 from Zenerdiode VR1 645 to the base of transistor Q3 657. The current flowingthrough R3 653 is proportional to the difference in voltage between thebase of transistor 657 Q3 and the input voltage V_(IN) 627. The resultis a current in the collector of transistor Q3 657 that decreasessubstantially linearly with increasing input voltage V_(IN) 627. Thecollector current in transistor Q3 657 produces a voltage drop throughresistor R1 649 such that the voltage, or programming signal S₁ 639, onthe gate of transistor Q2 633 is proportional to the weighted sum of theclamp voltage V_(CLAMP) 623 and the input voltage V_(IN) 627. The gatevoltage on the gate of transistor Q2 633 controls the current in thedissipative element transistor Q2 633 to adjust the clamp voltageV_(CLAMP) 623 at a desired value for a given V_(IN) 627.

A first order analysis using reasonable engineering approximationsreveals that the behavior of the circuit of power supply 601 isdescribed by the expression$V_{CLAMP} = {{V_{REF}\left( {1 + \frac{{R1} \cdot {R4}}{{R3}\left( {{R2} + {R4}} \right)}} \right)} - {V_{IN}\left( {2 - \frac{{R1} \cdot {R2}}{{R3}\left( {{R2} + {R4}} \right)}} \right)}}$that describes a substantially straight line on the graph of V_(CLAMP)versus V_(IN), as shown in FIG. 5. An engineer can select values forresistances R1 649, R2 651, R3 653 and R4 655 along with V_(REF) 641 toachieve the locus of desired operation as illustrated in FIG. 5.

FIG. 7 is a schematic illustrating one embodiment of an electricalcircuit such as for example a power supply 701 utilizing a dissipativeclamp network 711 in accordance with the teachings of the presentinvention. As shown in the depicted embodiment, diode D3 737 providesthe unidirectional path for energy from the primary winding 707 of thetransformer 709 to enter the network 711. Zener diode VR1 745 makes astable reference voltage V_(REF) 741 relative to the circuit inputnegative rail of input 721. In one embodiment, a bipolar PNP transistorQ2 733 is the principal dissipative element. In another embodiment, itis appreciated that other types of dissipative elements could be used inplace of a bipolar PNP transistor 733 such as for example a P channelMOSFET transistor. Resistor R1 753 is an optional additional dissipativeelement allowing the dissipated energy to be split between the bipolartransistor 733 and resistor R1 753. The energy is held by an energystorage element capacitor 735 and is lost through a dissipative elementstransistor 733 and resistor 753.

In operation, the sum of the voltages V_(IN) 727 across the input 721and V_(CLAMP) 723 across capacitor 735 is substantially constant. Thus,when V_(IN) 727 is relatively low, V_(CLAMP) 723 is relatively high.Conversely, when V_(IN) 727 is relatively high, V_(CLAMP) 723 isrelatively low. Accordingly, V_(CLAMP) 723 is responsive to V_(IN) 727received at input 721. Since the reference voltage V_(REF) 741 providedby zener diode VR1 745 is relative to the circuit input 721 negativerail, the operation of the clamp network 711 shown in FIG. 7 provides aclamp that limits V_(CLAMP) 723 across capacitor 735 to the locus ofdesired operation shown in FIG. 5. In another embodiment is itappreciated that zener diode VR1 745 reference voltage V_(REF) 741 couldbe achieved with several lower voltage zener diodes in series.

It is appreciated that in the illustrated embodiment, transistor 733 incombination with resistor 753 and diode 745 embody a sensing network tosense V_(IN) 727 and thereby regulate the voltage across capacitor 735such that the sum of V_(IN) 727 and V_(CLAMP) 723 remain substantiallyconstant during circuit operation. Accordingly, the voltage V_(SWITCH)725 across power switch Q1 703 is maintained below a voltage limit ofpower switch Q1 703 in accordance with the teachings of the presentinvention.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1-31. (canceled).
 32. A circuit, comprising: an energy transfer elementcoupled between an input of the circuit and an output of the circuit; asensing network coupled to the input of the circuit; a dissipativeelement coupled to the sensing network and coupled to the energytransfer element; and an energy storage element coupled to thedissipative element and coupled to the energy transfer element such thatenergy received from the energy transfer element is stored in the energystorage element, wherein the energy stored in the energy storage elementis dissipated through the dissipative element in response to the sensingnetwork.
 33. The circuit of claim 32 further comprising a unidirectionalpath of energy between the energy transfer element and the energystorage element.
 34. The circuit of claim 33 wherein the unidirectionalpath comprises a diode coupled between the energy transfer element andthe energy storage element.
 35. The circuit of claim 32 wherein theenergy storage element is an inductive element.
 36. The circuit of claim32 wherein the dissipative element is coupled to dissipate the energystored in the energy storage element to provide a clamp voltage acrossthe energy transfer element in response to the input of the circuit. 37.The circuit of claim 32 further comprising a power switch coupled to theenergy transfer element.
 38. The circuit of claim 37 wherein thedissipative element is coupled to dissipate energy stored in the energystorage element in response to the input of the circuit to maintain avoltage across the power switch below a power switch voltage limit. 39.The circuit of claim 32 wherein the energy storage element comprises acapacitor coupled to energy transfer element and coupled to thedissipative element.
 40. The circuit of claim 32 wherein the dissipativeelement comprises a transistor coupled to the energy storage element andcoupled to the sensing network.